The present invention relates to communication systems and more particularly to a system for monitoring a fiber optic transmission system using an optical side tone as a test signal.
In long distance fiber optic communication systems it is important to monitor the health of the system. For example, monitoring can be used to detect faults or breaks in the fiber optic cable, faulty repeaters or amplifiers or other problems with the system.
Prior art monitoring techniques include the use a testing system which generates a test signal and modulating the test signal onto a single channel (or wavelength) with the transmitted data signal. For example, the data signal may be amplitude modulated by the test signal. A loop-back coupler within an optical amplifier or repeater located downstream is used to return a portion of the transmitted signal (data signal plus test signal modulation) to the testing system. The testing system then separates the test signal from the data signal and processes the test signal to examine the health of the transmission system. U.S. Pat. Nos. 4,586,186 and 4,633,464 to Cleo Anderson et al. and assigned to AT&T disclose a similar technique to modulate test response information from a repeater onto the main data signal to monitor the health of the system.
There are several disadvantages and drawbacks, however, with the prior art monitoring systems. Due to ordinary losses or attenuation of signals in the fiber optic transmission systems (due to absorption, scattering, etc.), data signals must be amplified at periodic intervals. Because the test signal is modulated onto the data signal, the test signal modulation will further impair some of the bits of the data signal.
For example, FIG. 1A illustrates the test signal 12 and the data signal ("1"s and "0"s) prior to modulation. Initially, data bit 10 is above the threshold level 14 and therefore is a "1." As shown in FIG. 1B, amplitude modulating the data signal with the test signal can cause the amplitude of data bit 10 to significantly decrease. Due to this interference from the test signal modulation combined with the ordinary attenuation of the data signal, the amplitude of data bit 10 may decline below the threshold level for a "1," resulting in an erroneous bit (FIG. 1B). Consequently, prior art monitoring systems require additional optical amplifiers or repeaters to compensate for losses resulting from test signal modulation.
Moreover, the prior art monitoring systems described above are incapable of simultaneously monitoring more than a single channel. To increase information capacity, two or more wavelengths (or channels) may be used to carry information over the same optical fiber. This is known as wavelength-division multiplexing (WDM). The prior art monitoring technique described above uses a test signal having the same wavelength as the data signal, and therefore, can test only a single channel at a time. A separate testing system for each wavelength or channel would be required to simultaneously monitor multiple channels using the prior art technique. As a result, such a system would have a prohibitively high cost.
Therefore, a need has arisen for a monitoring system that does not attenuate data signals and which is capable of simultaneously monitoring multiple channels (wavelengths) on an optical fiber.